GB1202281A - Improvements in or relating to testing apparatus for gases - Google Patents

Abstract

1,202,281. Measuring current. COMPAGNIE DES COMPTEURS. Sept.8, 1967 [Sept.20, 1966; March 14, 1967, April 4, 1967, July 19, 1967 (2)], No.41162/67. Heading G1U. [Also in Division H4] In a system for determining the physical properties of a gas, oscillations are maintained in a gas-filled resonant cavity having a neckshaped opening by means of acoustic feedback between an energizing transducer and a pickup transducer. As shown, a loudspeaker 5 (or Piezo-electric transducer), microphone 6 and amplifier 8 maintain oscillations in a hemispherical resonator 1 of volume V and having a neck 4 of length L and cross-sectional area S. The oscillation frequency f is given by (where k is constant, y is the specific heat ratio of the gas, R is the gas constant, T is the gas temperature and M is the molecular weight of the gas). For a given resonator geometry and a given gas, f provides a measure of gas temperature or the velocity C of sound in the gas (C equals the facor under the second square root sign) and can provide indirectly a measure of the current flowing through an electric heating element disposed in the resonator. In an embodiment for determining when the contamination of one gas by another (e.g. air by fire-damp) reaches a critical level, a band-pass filter 11 in the feedback circuit allows oscillations to be maintained only when that level is reached, and negative temperature coefficient resistances are provided in the filter circuit to make this determination independent of temperature. Alternatively the temperature independence is obtained by maintaining the ratio TS/VL constant by using a bi-metal strip (10) either to (a) operate a shutter (9), Fig. 2 (not shown) movable over the neck opening 4 to keep T.S. constant, (b) operate a bellows (9a Fig.3) coupled to the resonator to keep T/V constant, or (c) operate a tube (9b), Fig.4 (not shown) slidable in the neck opening 4 to keep T/L constant. To deaden vibrations of the resonator wall the wall may be made of rubber or of metal covered inside and outside with rubber. In a second embodiment Fig.7 for determining gas composition one resonator 19 is supplied with a reference gas and a second resonator 20 is supplied with a gas to be analysed. Both gases are piped through a thermally-insulated enclosure so that they are supplied at the same temperature. A signal proportional to the difference between the oscillation frequencies F 1 , F 2 of the two resonators is provided by a phase comparator 21. If this signal reaches a given level, corresponding to a given contamination of the gas in resonator 20, a gate 22 is opened to apply an audible frequency signal (e.g. from resonator 20) to an alarm 23 comprising an amplifier and loudspeaker. In a modification resonator 19 is replaced by an electrical oscillator whose frequency . is temperature dependent in the same way as the resonator frequency. Modifications of the transducer positioning in relation to the resonant cavity 1 are described.